Overview of recent ambient noise measurements in Croatia in free-field and in buildings

The paper describes some recent applications of the measurements of ambient vibrations (microseismic noise) in Croatia. They include free-field measurements, as well as those done within buildings. Data obtained in the field at the studied localities are consistent with the properties of shallow geological structures known to exist there. In Zagreb, Horizontal-to-Vertical Spectral Ratios (HVSR) indicate thick alluvial cover (over 100 m) that gradually gets thinner towards the slopes of the Mt. Medvednica. A similar situation – on a smaller scale – is also encountered in Ston, where HVSR profiles reveal several tens of meters thick sedimentary cover over the bedrock which gets exposed at the Bartolomija hill. Analyses of records from Ston and Dubrovnik suggest that soil-building resonance must be seriously considered. Measurements in the buildings were analysed by a newly developed software package FREDA – a suite of Matlab routines. Based on tests on synthetic and real data it is concluded that spectral methods are in general more robust and less dependent on parameters of the algorithm employed, than time-domain analyses. Spectral algorithms are also better in resolving higher modes. Applications to real buildings proved that analyses of shaking induced by ambient vibrations in most cases lead to well constrained, reliable, and time independent estimates of frequencies and damping of the buildings’ vibrational modes for small excitation levels.